The invention relates to a method and apparatus for screening relatively fine material from a mixture of material, particularly for use on a dynamically balanced vibrating conveyor apparatus.
In materials processing systems, vibrating conveyors are commonly used to carry material from one operation to the next. For some operations to be performed effectively, the material being conveyed often requires a separation process in which fine material is removed from coarser material. One example in the forest products industry relates to feeding waste wood to chippers which convert the waste wood to chips used in the pulp and paper industry. It is useful to remove fine materials such as sawdust, dirt and small pieces of wood from the waste wood before chipping to minimize the amount of material passing through the chipper. This removal of fines minimizes the amount of fine material to be screened from the chips after the chipping operation, and also lessens the dulling effect of the fine material on the chipper knives.
Usually, separation of fine material is performed by a simple mechanical screen which is installed in a pan of a vibrating conveyor, the screen comprising either a flat plate with openings, or a woven or welded wire screen. Either of these types of screens suffers from at least two deficiencies. Firstly, the holes in the screen tend to become plugged with pieces of wood which are sufficiently small to enter the openings, but are too large to pass right through the openings. Secondly, weight of wood being conveyed along the conveyor and over the screen can deform the screen and eventually break the screen.
Various attempts have been made to improve conventional screening as described above. Two such attempts are disclosed in U.S. Pat. No. 4,504,386 issued to Dyren et al, and U.S. Pat. No. 4,664,790 issued to Lundqvist. Both of these patents disclose screening structure for vibrating conveyors, in which the screening structure has two sets of elongated elements, for example rods, bars, etc. which extend axially along a portion of the conveyor. Elements of each set alternate with each other and are spaced transversely apart to provide elongated gaps therebetween. Material passing along the conveyor that is narrower than the gaps falls through the gaps, whereas material that is larger than the gaps is conveyed across the rods to the end of the conveyor. Because both of the above screening assemblies rely on elongated gaps for separation, long thin strips of wood can pass through the openings, which is undesirable in some processing operations. Furthermore, in Dyen et al, vibrations are applied to only one of the sets of elongated members and the remaining set is stationary which could detract from efficient displacement of material along the conveyor. In Lundqvist, a first set of members is vibrated with a conveyor, whereas the second set of members can be vibrated with a separate vibrating system, whose frequency and phase can be varied with respect to the vibration of the conveyor. While the vibration of the first set, being associated with the conveyor, would appear to result in translation of material, vibrations of the second set appear to be separate from translation, and thus would not contribute to useful movement of material along the conveyor.
A further screen structure for wood chips is shown in U.S. Pat. No. 4,660,726, issued to Woode, which discloses two sets of reciprocating bars having serrated tops, the bars being mounted for out-of-phase vibration in a generally horizontal plane. There does not appear to be any attempt to feed the material across the screen by a normal vibrating conveyor motion, but instead the screen is sloped so that material is conveyed by the action of gravity.
From the above it is seen that prior art two-set screening systems incorporated into vibrating conveyors would appear to pass elongated thin material, which is not always acceptable, and commonly the screens detract from the normal conveying motion due to the mode of vibration of one or both of the sets of screen material. Furthermore, in applications where both sets of screen materials are vibrated, a separate vibrating system is required, increasing cost and complexity of the structure.